Solid-state image pickup device having a level shift unit and signal processing unit where a voltage range supplied to the level shift unit is wider than a voltage range supplied to the signal processing unit

ABSTRACT

Dark current from a transfer transistor is suppressed and power-supply voltage in a second semiconductor substrate is lowered. A solid-state image pickup device includes a pixel array, a plurality of common output lines receiving signals read out from a plurality of pixels, a transfer scanning unit sequentially driving the plurality of transfer transistors, a signal processing unit processing the signals output to the common signal lines, and a level shift unit making amplitude of a pulse supplied to a gate of the transfer transistor larger than amplitude of a pulse supplied to a gate of a transistor constituting the signal processing unit. The pixel array and the level shift unit are arranged on a first semiconductor substrate, whereas the plurality of common output lines and the signal processing unit are arranged on a second semiconductor substrate.

TECHNICAL FIELD

The present invention relates generally to solid-state image pickupdevices. More particularly, the present invention relates to asolid-state image pickup device including a photoelectric conversionunit and a peripheral circuit unit separately formed on differentsubstrates.

BACKGROUND ART

For solid-state image pickup devices, a configuration is known in whicha photo-electric conversion unit and a peripheral circuit unitseparately formed on different substrates are electrically connectedthrough a micro bump. Patent Literature 1 discloses the followingconfiguration. A photoelectric conversion unit is arranged on a firstsemiconductor substrate, whereas a peripheral circuit for reading out asignal from the photoelectric conversion unit, such as a verticalscanning unit and a horizontal scanning unit, is arranged on a secondsemiconductor substrate. Leakage current is less problematic fortransistors arranged on the second semiconductor substrate than for apixel unit. Accordingly, gate insulating films of the downsizedtransistors on the second semiconductor substrate are thinned with aCMOS process.

Patent Literature 1 does not fully discuss amplitude of a pulse fordriving a transfer transistor that transfers charge generated in thephotoelectric conversion unit. The pulse fed to a gate of the transfertransistor may be desired to have larger amplitude than those fed toother transistors in the pixel unit or transistors in the peripheralcircuit because of the following reason.

Current leaking from a channel of the transfer transistor during acharge accumulation period of the photoelectric conversion unit mayresult in dark current. To prevent the dark current, voltage fed to thegate of the transfer transistor during the charge accumulation periodcan be set to be lower (higher regarding a PMOS transistor) thanoff-voltage of the other transistors. Accordingly, a range of thevoltage fed to the gate of the transfer transistor is wider than that ofthe voltage fed to the other transistors in the pixel unit and thetransistors in the peripheral circuit.

However, Patent Literature 1 does not disclose a circuit for making theamplitude of the pulse fed to the transfer transistor larger than thatof the pulses fed to the other transistors and, thus, does not discusswhere to arrange such a circuit. Depending on the arrangement,realization of low power-supply voltage is disturbed.

In view of the foregoing problem, the present invention suppresses darkcurrent from the transfer transistor and lowers power-supply voltage ina second semiconductor substrate at the same time.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Laid-Open No. 2009-170448

SUMMARY OF INVENTION

In view of the foregoing problem, a solid-state image pickup deviceincludes: a pixel array including a plurality of pixels each of whichincludes a photoelectric conversion unit and a transfer transistorconfigured to transfer charge generated in the photoelectric conversionunit; a plurality of common output lines configured to receive signalsfrom the plurality of pixels included in the pixel array; a transferscanning unit configured to sequentially drive the plurality of transfertransistors; a signal processing unit configured to process the signalsoutput to the common signal lines; and a level shift unit configured tomake amplitude of a pulse supplied to a gate of the transfer transistorlarger than amplitude of a pulse supplied to a gate of a transistorconstituting the signal processing unit. The pixel array and the levelshift unit are arranged on a first semiconductor substrate, whereas theplurality of common output lines and the signal processing unit arearranged on a second semiconductor substrate.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are diagrams illustrating examples of an equivalentcircuit of a pixel in accordance with an exemplary embodiment of thepresent invention.

FIGS. 2A and 2B are plan views of a solid-state image pickup device inaccordance with a first exemplary embodiment of the present invention.

FIGS. 3A and 3B are plan views of a solid-state image pickup device inaccordance with a second exemplary embodiment of the present invention.

FIGS. 4A and 4B are plan views of a solid-state image pickup device inaccordance with a third exemplary embodiment of the present invention.

FIG. 5 is a sectional view for illustrating an electric connection inthe solid-state image pickup device in accordance with the firstexemplary embodiment of the present invention.

FIG. 6 is a diagram illustrating an example of an equivalent circuit ofa solid-state image pickup device in accordance with an exemplaryembodiment of the present invention.

FIG. 7 is a diagram of an example of an equivalent circuit of a levelshift unit in accordance with an exemplary embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

A description will now be given for a configuration of a pixel includedin a solid-state image pickup device in accordance with an exemplaryembodiment of the present invention.

FIGS. 1A and 1B are diagrams each illustrating an equivalent circuit ofa pixel “pix” in accordance with an exemplary embodiment of the presentinvention. Although FIGS. 1A and 1B illustrate one pixel, a plurality ofpixels constitute a pixel array in fact.

Holes and electrons result from photoelectric conversion by aphotoelectric conversion unit 101. For example, a photodiode serves asthe photoelectric conversion unit 101.

A transfer unit 102, such as a MOS transistor (i.e., a transfer MOStransistor), transfers the charge generated in the photoelectricconversion unit 101.

A node 103 serves as a floating unit for signal readout. Potential ofthe floating unit shifts into a floating state when the transfer unit102 transfers the charge of the photoelectric conversion unit 101thereto. The floating unit 103 includes, for example, a floatingdiffusion (hereinafter, abbreviated as an FD). The FD is arranged on afirst semiconductor substrate.

A reset unit 104, such as a MOS transistor (i.e., a reset MOStransistor), sets at least the potential of the floating unit 103 toreference potential. The reset unit 104 turns on at the same time withthe transfer unit 102 to set potential of the photoelectric conversionunit 101 to the reference potential.

An amplifying unit 105 amplifies a signal based on one of the chargepair generated in the photoelectric conversion unit 101 and outputs theamplified signal. For example, a MOS transistor is used as theamplifying unit 105. In such a case, a gate of the MOS transistorserving as the amplifying unit 105 (i.e., an amplifying MOS transistor)is electrically connected to the FD.

A transfer control line 106 is for controlling an operation of thetransfer unit 102. A reset control line 107 is for controlling anoperation of the reset unit 104. When MOS transistors constitute thetransfer unit 102 and the reset unit 104, the transfer control line 106and the reset control line 107 supply gates of the MOS transistors withpulses for turning on and off the MOS transistors, respectively.

The signal amplified by the amplifying unit 105 is output to a commonoutput line 108. That is, signals are read out to the common output line108 from the plurality of pixels included in the pixel array.

A constant-current source 109 supplies bias current to the amplifyingunit 105. In this circuit configuration, the constant-current source 109supplies the bias current to the amplifying MOS transistor so that theamplifying MOS transistor operates as a source follower.

Referring to FIG. 1A, a voltage V1 is supplied to drains of theamplifying MOS transistor 105A and the reset MOS transistor 104A.Although the voltage is commonly supplied here, the voltage may besupplied from different power supplies. A voltage V2 is supplied to theconstant-current source 109A illustrated in FIG. 1A.

Referring to FIG. 1B, a voltage V3 is supplied to a drain of the resetMOS transistor 104B, whereas a voltage V4 is supplied to a drain of theamplifying MOS transistor 105B. A voltage V5 is supplied to theconstant-current source 109B illustrated in FIG. 1B.

A dotted line pixA indicates the pixel-constituting units arranged onthe first semiconductor substrate, whereas a dotted line pixB indicatesthose arranged on a second semiconductor substrate. The elementsenclosed by the dotted lines pixA and pixB constitute the pixel pix.

Differences between FIG. 1A and FIG. 1B will now be described. Differentelements are distinguished from one another by attaching characters “a”and “b” to reference characters. More specifically, each of theamplifying MOS transistor and the reset MOS transistor has a differentconductivity type. N-channel metal-oxide semiconductor (NMOS)transistors are used in FIG. 1A, whereas p-channel MOS (PMOS)transistors are used in FIG. 1B. To cope with the different conductivitytype, the voltages supplied to the respective transistors and theconstant-current source also differ.

Referring to FIG. 1A, the voltage V1 corresponds to a power-supplyvoltage, such as 5 V or 3.3 V. The voltage V2, such as ground potential,is lower than the power-supply voltage V1. Referring to FIG. 1B, thevoltages V3 and V4, such as the ground potential, are relatively low,whereas the voltage V5, such as 3.3 V or 1.8 V, is higher than thevoltage V3.

In FIG. 1B, a PMOS transistor serves as the amplifying MOS transistor.The photoelectric conversion unit 101 uses electrons as signal charge. Asufficiently large amount of incident light causes gate potential of thePMOS transistor to drop. In response to the gate potential drop, sourcepotential of the PMOS transistor changes in a rising direction comparedwith that in a dark state. That is, the common output line 108 can bedriven with a high driving power when signal amplitude is larger thanthat in a reset state. Thus, the configuration in FIG. 1B has moreadvantages readout speed than the configuration in FIG. 1A. In therelated art, since such a configuration is arranged on a singlesemiconductor substrate, a structure of the resulting pixelunfortunately complicates because of use of different wells in thepixel. In contrast, such a configuration is separately arranged ondifferent substrates in an exemplary embodiment of the presentinvention, whereby the disadvantage of the complex structure can beovercome. Furthermore, the configuration in FIG. 1B can have a narroweroperating-voltage range and is advantageous from a standpoint ofrealization of low power-supply voltage.

What is important here is not the use of the PMOS transistor serving asthe amplifying MOS transistor but is the use of a MOS transistor havinga polarity opposite to that of the signal charge. More specifically,PMOS transistors are used as the amplifying MOS transistor and the resetMOS transistor when electrons serve as the signal charge, whereas NMOStransistors are used when holes serve as the signal charge. That is,when the transfer MOS transistor has a first conductivity type, theamplifying MOS transistor and the reset MOS transistor have a secondconductivity type opposite to the first one.

Although the configuration of the pixel has been described above, theconfiguration of the pixel is not limited to the described one. Forexample, a junction field effect transistor (JFET) may be used as theamplifying transistor. Additionally, the photoelectric conversion unit101 may use holes as the signal charge. In such a case, a PMOStransistor is used as the transfer transistor. In addition, a pluralityof photoelectric conversion units may share the amplifying transistorand the reset transistor. A select transistor connected to theamplifying transistor in series may be additionally used. The elementsof the pixel are not necessarily separately arranged on the plurality ofsemiconductor substrates in accordance with the example configurationdescribed above. In addition to the foregoing example, the reset MOStransistor and the amplifying MOS transistor may be arranged on thefirst semiconductor substrate. However, the common output line 108 canbe arranged in the second semiconductor substrate. Furthermore, theamplifying MOS transistor and the reset MOS transistor may be omittedfrom the pixel and the transfer MOS transistor may output the chargegenerated in the photoelectric conversion unit 101 directly to thecommon output line 108.

Features of the present invention will now be described below throughexemplary embodiments. In an example described below, MOS transistorsare used as the transfer unit 102, the reset unit 104, and theamplifying unit 105 and electrons serve as the signal charge. When holesare used as the signal charge, a conductivity type of each pixelsemiconductor region arranged on the first semiconductor substrate maybe at least reversed.

First Embodiment

In a first exemplary embodiment, a photoelectric conversion unit, atransfer MOS transistor, and an FD constituting each pixel are arrangedon a first semiconductor substrate. Furthermore, a transfer scanningunit for sequentially driving the transfer MOS transistors and atransfer buffer unit for buffering a driving signal from the transferscanning unit are arranged on the first semiconductor substrate. Thetransfer buffer unit is arranged in a path between gates of the transferMOS transistors and the transfer scanning unit. The transfer buffer unitincludes a level shift unit therein. The level shift unit has a functionfor making amplitude of a pulse supplied to the gate of the transfer MOStransistor larger than amplitude of a pulse supplied to a gate of atransistor constituting at least one of a reset scanning unit and asignal processing unit arranged on a second semiconductor substrate.

A reset MOS transistor and an amplifying MOS transistor constitutingeach pixel are arranged on the second semiconductor substrate.Furthermore, the reset scanning unit for driving the reset MOStransistors and a reset buffer unit for buffering a driving signal fromthe reset scanning unit are arranged on the second semiconductorsubstrate. The reset buffer unit is arranged in a path between gates ofthe reset MOS transistors and the reset scanning unit. Moreover, atiming generator for supplying a clock pulse to each scanning unit andthe signal processing unit for processing signals output to a commonoutput line are arranged on the second semiconductor substrate.

FIGS. 2A and 2B illustrate a specific layout example on semiconductorsubstrates in accordance with the first exemplary embodiment.

More specifically, FIG. 2A illustrates a layout on a first semiconductorsubstrate 201A, whereas FIG. 2B illustrates a layout on a secondsemiconductor substrate 201B. A positional relationship between thearranged first and second semiconductor substrates generally matches theone illustrated in FIGS. 2A and 2B.

A pixel array portions 202A and 202B collectively indicate a pixelarray. The pixel array portion 202A includes a photoelectric conversionunit, a transfer MOS transistor, and an FD of elements constituting eachpixel, whereas the pixel array portion 202B includes a reset MOStransistor and an amplifying MOS transistor of the elements constitutingeach pixel.

A transfer buffer unit 203A, a reset buffer unit 203B, a transferscanning unit 204A, and a reset scanning unit 204B are also illustrated.The transfer scanning unit 204A and the reset scanning unit 204B supplydriving pulses to the pixels in the pixel array. More specifically, eachof the transfer scanning unit 204A and the reset scanning unit 204Bincludes a shift register and an address decoder and desirably has atleast a function for sequentially selecting a plurality of pixels in thepixel array. The pulses output from the transfer scanning unit 204A andthe reset scanning unit 204B undergo impedance conversion in thetransfer buffer unit 203A and the reset buffer unit 203B, respectively,before being supplied to the pixels.

A transfer control line 205A carries the driving pulse output from thetransfer buffer unit 203A, whereas a reset control line 205B carries thedriving pulse output from the reset buffer unit 203B.

A timing generator 206 is capable of supplying a clock signal to eachscanning unit and each buffer unit.

A signal processing unit 207 is, for example, a circuit capable ofprocessing signals from a plurality of pixels in parallel. Morespecifically, the signal processing unit 207 includes an amplifier, acorrelated double sampling (CDS) circuit, and an analog/digitalconverter provided for each pixel column. The signal processing unit 207includes a plurality of transistors each functioning as a switch or partof an operational amplifier.

The signal processing unit 207 processes the signals in parallel toconvert the signals into a serial signal. The serial signal is read outto outside through a horizontal output line 208.

A horizontal scanning unit 209 sequentially supplies a pulse so that thesignals processed by the signal processing unit 207 are converted intothe serial signal. Just like the scanning units, the horizontal scanningunit 209 desirably has at least a function for sequentially selectingone of the signals processed by the signal processing unit 207 inparallel.

An amplifying unit 210 amplifies the signal output from the horizontaloutput line 208 and outputs the amplified signal to outside. Theamplifying unit 210 is provided as needed.

The FD arranged on the first semiconductor substrate 201A iselectrically connected to a gate of the amplifying MOS transistorarranged on the second semiconductor substrate 201B. Additionally, thetransfer scanning unit 204A is connected to the timing generator 206 tobe able to receive the clock pulse from the timing generator 206.

FIG. 5 illustrates a sectional view for describing an electricalconnection between a first semiconductor substrate and a secondsemiconductor substrate.

A first semiconductor substrate 501A includes a transfer buffer unit503A and a transfer scanning unit 504A, whereas a second semiconductorsubstrate 501B includes a reset buffer unit 503B and a reset scanningunit 504B. A timing generator 506 generates a timing signal forcontrolling operations of the transfer scanning unit 504A and the resetscanning unit 504B. An n-type semiconductor region 507 is capable ofstoring electrons generated in a photoelectric conversion unit. Thephotoelectric conversion unit including the n-type semiconductor region507 constitutes, for example, a pinned photodiode.

A gate electrode 508 is arranged over the first semiconductor substrate501A through an insulating film. In accordance with a supplied pulse,the gate electrode 508 transfers the charge accumulated in the n-typesemiconductor region 507 to an FD 509 formed of an n-type semiconductorregion. The FD 509 receives the electrons generated in and transferredfrom the photoelectric conversion unit.

Element isolation regions 510, 517, 520, 523, 526, 529, and 533 isolateadjacent elements from one another to prevent a channel between theadjacent elements.

A contact plug 511 is electrically connected to the FD 509. The contactplug 511 is formed by filling a contact hole on the FD 509 with aconductive material.

An electrical contact 512 electrically connects the first semiconductorsubstrate 501A to the second semiconductor substrate 501B. Morespecifically, the electrical contact 512 electrically connects the FD509 arranged on the first semiconductor substrate 501A to a source of areset MOS transistor arranged on the second semiconductor substrate501B. A contact plug 513 is formed by filling a contact hole on a sourceregion 514 of the reset MOS transistor with a conductive material.

The source region 514 of the reset MOS transistor is formed of an n-typesemiconductor region in the equivalent circuit of the pixel illustratedin FIG. 1A, whereas the source region 514 is formed of a p-typesemiconductor region in the equivalent circuit of the pixel illustratedin FIG. 1B. A gate electrode 515 is arranged over the secondsemiconductor substrate 501B through an insulating film. The gateelectrode 515 controls conduction of the reset MOS transistor inaccordance with a supplied pulse. A predetermined voltage is supplied toa drain region 516 of the reset MOS transistor to turn on the reset MOStransistor, so that the charge of the FD 509 is emitted. Furthermore,the transfer MOS transistor may be turned on at the same time with thereset MOS transistor, so that the charge of the n-type semiconductorregion 507 is also emitted.

A MOS transistor constituting the transfer buffer unit 503A has sourceand drain regions 518 and a gate electrode 519. Although FIG. 5illustrates an NMOS transistor having the same conductivity type as thepixel unit, a PMOS transistor on an n-type well may be further included.

A MOS transistor constituting the reset buffer unit 503B has source anddrain regions 521 and a gate electrode 522. Just like the transferbuffer unit 503A, the reset buffer unit 503B may further include a MOStransistor having the opposite conductivity type.

A MOS transistor constituting the transfer scanning unit 504A has sourceand drain regions 524 and a gate electrode 525. Just like the bufferunits, the transfer scanning unit 504A may further include a MOStransistor having the opposite conductivity type.

A MOS transistor constituting the reset scanning unit 504B has sourceand drain regions 527 and a gate electrode 528. Just like the transferscanning unit 504A, the reset scanning unit 504B may further include aMOS transistor having the opposite conductivity type.

An electrical contact 530 electrically connects the timing generator 506to the transfer scanning unit 504A and the reset scanning unit 504B.

A MOS transistor constituting the timing generator 506 has source anddrain regions 531 and a gate electrode 532. Just like theabove-described members, the timing generator 506 may further include aMOS transistor having the opposite conductivity type.

Although FIG. 5 omits illustration of the amplifying MOS transistor, theamplifying MOS transistor may be provided for each photoelectricconversion unit or each plurality of photoelectric conversion units. Inthis case, the amplifying MOS transistor is arranged on the secondsemiconductor substrate 501B. A gate of the amplifying MOS transistor isconnected to the electrical contact 512.

FIG. 6 illustrates a block diagram of a transfer scanning unit, a resettransfer unit, a transfer buffer unit, and a reset buffer unit. Althoughthe transfer scanning unit and the reset scanning unit are collectivelyillustrated as a single circuitry, the transfer scanning unit and thereset scanning unit are separately arranged on first and secondsemiconductor substrates, respectively.

The transistors constituting the scanning units and the buffer unitsillustrated in FIG. 5 constitute part of the circuitry illustrated inFIG. 6.

A transfer buffer unit 603A includes an AND circuit and a level shiftunit 605 for changing amplitude of a pulse from the transfer scanningunit. A signal “ptx” is input to an input terminal of the AND circuit,whereas the pulse from the transfer scanning unit is input to anotherinput terminal thereof. The AND circuit determines a logical AND of theinput signals and supplies a resulting driving pulse to a gate of atransfer MOS transistor.

A reset buffer unit 603B includes an AND circuit. A signal “pres” isinput to an input terminal of the AND circuit, whereas the pulse fromthe reset scanning unit is input to another input terminal thereof. TheAND circuit determines a logical AND of the input signals and supplies aresulting driving pulse to a gate of a reset MOS transistor.

The signals “ptx” and “pres” may be supplied to the first and secondsemiconductor substrates, respectively, directly from outside or throughthe timing generator.

Each of the scanning units 604 is constituted by a shift registerincluding a D flip-flop. The scanning units 604 operate in accordancewith pulses “start”, “clk”, and “reset” supplied from the timinggenerator.

The level shift unit 605 will now be described. FIG. 7 illustrates anequivalent circuit thereof.

The level shift unit 605 includes a PMOS transistor 71 and an NMOStransistor 72. The PMOS transistor 71 has a drain connected to a gate ofthe transfer MOS transistor through an output node 74 and the transfercontrol line and a source supplied with a voltage VA of, for example,5V. The NMOS transistor 72 has a drain connected to the gate of thetransfer MOS transistor through the output node 74 and the transfercontrol line and a source supplied with a reference voltage VB. A signof the reference voltage VB is opposite to that of the voltage VA. Morespecifically, the reference voltage VB may be, for example, −1.2 V. Thedrain of the PMOS transistor 71 is connected to the drain of the NMOStransistor 72. Gates of the PMOS transistor 71 and the NMOS transistor72 are connected to an input node 73. The input node 73 is supplied witha pulse having amplitude smaller than a result of VA-VB.

A case where the PMOS transistor 71 is turned off will now be discussed.A difference voltage Vgs71 between the voltage VA and the referencevoltage VB is generated between the gate and the source of the PMOStransistor 71. For example, when the voltage VA and the referencevoltage VB are equal to 5 V and −1.2 V, respectively,Vgs71=5 V−(−1.2 V)=6.2 V  Equation 1is generated. That is, the voltage of 6.2 V is applied to the gateinsulating film.

Since the voltage higher than that applied to transistors operatingbetween the power-supply voltage and the ground potential is applied tothe gate insulating film, the PMOS transistor 71 desirably have aconfiguration, such as a thick gate insulting film, different from theMOS transistors on the second semiconductor substrate.

Similarly, since the voltages of 5V and −1.2 V are applied to the gateof the transfer MOS transistor at the time of ON and OFF, respectively,a voltage of 6.2 V is applied to the gate insulting film of the transferMOS transistor. Accordingly, the transfer MOS transistor desirably hasthe different configuration based on the same reason as that for the MOStransistor of the transfer buffer unit.

When the buffer unit is arranged on the second semiconductor substrate,a widest range of voltage supplied to the transistor of the buffer unitcontrols thickness of the gate insulating film. Accordingly, the gateinsulting film of each MOS transistor arranged on the secondsemiconductor substrate has the thickness similar to that of the gateinsulting film of the MOS transistor arranged on the first semiconductorsubstrate. Such a configuration makes it difficult to realize anoperation at low voltage.

Collectively arranging the transfer MOS transistor and the MOStransistor of the transfer buffer unit on the first semiconductorsubstrate permits the second semiconductor substrate to include only theMOS transistors intended for voltage lower than that of the firstsemiconductor substrate. By arranging the buffer unit on the firstsemiconductor substrate, the transistors having a wide supplied voltagerange can be collected in the first semiconductor substrate and, thus,the gate insulting films of the transistors on the second semiconductorsubstrate can be thinned. That is, every transistor arranged on thesecond semiconductor substrate can be set to operate at low power-supplyvoltage. More specifically, when the circuit illustrated in FIG. 1A isused, an operating voltage range for the first semiconductor substrateis equal to 6.2 V, whereas that for the second semiconductor substratecan be set equal to 5 V or 3.3 V (=V1−V2).

Furthermore, when the circuit illustrated in FIG. 1B is used, the rangeof the voltage supplied to the MOS transistors on the secondsemiconductor substrate can be further narrowed. More specifically, useof PMOS transistors serving as the reset MOS transistor and theamplifying MOS transistor can further narrow the voltage range. Therange of the voltage supplied to the gate of the transfer MOS transistoris 6.2 V (Vtx is more than or equal to −1.2 V and less than or equal to5 V). In contrast, the range of the voltage supplied to the gate of thereset MOS transistor can be narrowed to 1.7 V (Vres is more than orequal to 3.3 V and less than or equal to 5 V).

The configuration according to the first exemplary embodiment cansuppress leakage of charge to the photoelectric conversion unit from thechannel of the transfer transistor and can lower power-supply voltage inthe second semiconductor substrate.

Second Embodiment

A second exemplary embodiment differs from the first one in that atransfer scanning unit is arranged on the second semiconductorsubstrate. Other than this point, the second exemplary embodiment issimilar to the first one. FIGS. 3A and 3B illustrates layouts ofelements on respective semiconductor substrates. Like referencecharacters are attached to parts having similar functions as thosedescribed in the first exemplary embodiment to omit a detaileddescription thereof.

As described in the first exemplary embodiment, the transfer scanningunit is desirably capable of at least sequentially outputting a drivingpulse and does not have to supply voltage of a wide range. Accordingly,integrating the transfer scanning unit and a reset scanning unit into asingle scanning unit 204B and arranging the common scanning unit 204B onthe second semiconductor substrate can reduce a circuit area andincrease an area of a photoelectric conversion unit.

FIGS. 4A and 4B illustrate an alteration of this exemplary embodiment.The common scanning unit is arranged on the second semiconductorsubstrate in FIGS. 3A and 3B, whereas a common scanning unit 404 may bearranged on the first semiconductor substrate as illustrated in FIGS. 4Aand 4B.

Although specific exemplary embodiments of the present invention havebeen described above, the present invention should not be limited tothese exemplary embodiments and can be modified without departing fromthe spirit of the present invention.

In the exemplary embodiments, the case has been described in which thevoltage supplied to the gate of the transfer MOS transistor during anOFF period is lower than the off-period voltage supplied to other MOStransistors. However, the present invention can be applied as long asthe voltage supplied to the gate of the transfer MOS transistor duringan ON period is higher than the on-period voltage supplied to the otherMOS transistor.

Although the photoelectric conversion unit, the transfer MOS transistor,and the FD of the pixel are arranged on the first semiconductorsubstrate, the pixel configuration is not limited to this example. Atleast one of the amplifying MOS transistor and the reset MOS transistormay be arranged on the first semiconductor substrate. Furthermore, whenthe pixel further includes a select MOS transistor, the select MOStransistor may also be arranged on the first semiconductor substrate.

The description has been given for the configuration in which the levelshift unit is included in the transfer buffer unit arranged between thetransfer scanning circuit and the gate of the transfer MOS transistor.However, the configuration of the level shift unit is not limited tothis example. The transfer scanning unit may use a circuit for largepulse amplitude. In such a case, the transfer scanning unit includes thelevel shift unit.

The level shift unit desirably has at least a function of making thevoltage range of the pulse supplied to the gate of the transfertransistor wider than the voltage range of the pulse supplied to thegates of the transistors constituting at least one of the reset scanningunit and the signal processing unit arranged on the second semiconductorsubstrate.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2009-274964, filed Dec. 2, 2009, which is hereby incorporated byreference herein in its entirety.

REFERENCE SIGNS LIST

101 Photoelectric conversion unit

102 Transfer transistor

108 Common output line

204A Transfer scanning unit

207 Signal processing unit

605 Level shift unit

The invention claimed is:
 1. A solid-state image pickup devicecomprising: a pixel array including a plurality of pixels, each of theplurality of pixels including a photoelectric conversion unit and atransfer transistor configured to transfer charge generated in thephotoelectric conversion unit; a plurality of common output linesconfigured to receive signals generated in the plurality of pixelsincluded in the pixel array; a transfer scanning unit configured tosequentially drive the plurality of transfer transistors; a signalprocessing unit configured to process the signals generated in theplurality of pixels; and a level shift unit, arranged at an electricalpath between the transfer scanning unit and the transfer transistor,configured to receive a pulse from the transfer scanning unit andincrease an amplitude of the pulse and supply a gate of the transfertransistor with the pulse having the increased amplitude, wherein thepixel array and the level shift unit are arranged on a firstsemiconductor substrate, whereas the plurality of common output lines,the signal processing unit, and the transfer scanning unit are arrangedon a second semiconductor substrate, and wherein a voltage rangesupplied to the level shift unit is wider than a voltage range suppliedto the signal processing unit, wherein a gate insulating film of atransistor arranged on the level shift unit is thicker than a gateinsulating film of a transistor arranged on the signal processing unit.2. The solid-state image pickup device according to claim 1, wherein thepixel further includes an amplifying unit configured to amplify a signalbased on the charge generated in the photoelectric conversion unit. 3.The solid-state image pickup device according to claim 1, furthercomprising: a transfer buffer unit in a path between the transfertransistors and the transfer scanning unit, the transfer buffer unitbuffering a signal from the transfer scanning unit and including thelevel shift unit.
 4. The solid-state image pickup device according toclaim 1, each of the plurality of pixels further including a reset unit,the solid-state image pickup device further comprising: a reset scanningunit configured to sequentially drive the plurality of reset units; anda reset buffer unit in a path between the reset units and the resetscanning unit, the reset buffer unit buffering a signal from the resetscanning unit.
 5. The solid-state image pickup device according to claim3, each of the plurality of pixels further including a reset unit, thesolid-state image pickup device further comprising: a reset scanningunit configured to sequentially drive the plurality of reset units, andwherein the transfer scanning unit and the reset scanning unit areintegrated.
 6. The solid-state image pickup device according to claim 2,wherein each of the plurality of pixels includes a reset transistor, andwherein the transfer transistor is a MOS transistor having a firstconductivity type, the reset transistor is a MOS transistor having asecond conductivity type, and the amplifying unit includes a sourcefollower constituted by a transistor having the second conductivitytype.